safe-search: check for changed IP addresses weekly

safe-search: check for changed IP addresses weekly

The current default of hourly is too fast. Some services such as
DuckDuckGo return IPs from a pool based on the user's location instead
of a fixed IP address. This change prevents unnecessary writes to the
flash memory by only updating once per week.

Signed-off-by: Gregory L. Dietsche <gregory.dietsche@cuw.edu>